The present invention relates to a membrane for the separation of hydrogen from a gas mixture; a methanol reformation system equipped with such a membrane; and a method for operating such a system.
A known technique for obtaining highly pure hydrogen consists in producing a gas mixture containing hydrogen from suitable starting substances by means of a corresponding chemical reaction and then separating the hydrogen from this mixture using a hydrogen separating membrane. The hydrogen separating membrane is designed so that only hydrogen gas can diffuse through it. An important area of application consists of reformation systems for obtaining hydrogen, for example, systems for steam reformation of methanol.
As a result of the reformation reaction of a suitable hydrocarbon, including hydrocarbon derivatives such as methanol, a reformate gas containing hydrogen is produced from which the hydrogen is separated using a hydrogen separation stage. The hydrogen separation stage contains a hydrogen separating membrane as an essential element. In methanol reformation systems, as disclosed in U.S. Pat. Nos. 4,981,676 and 5,229,102, the hydrogen separation stage is integrated into the reformation reactor stage so that the hydrogen separating membrane forms a side wall of the reactor that delimits the reformation reaction chamber. Alternatively, the hydrogen separation stage can also be in the form of an independent unit connected downstream from the reactor stage.
Recently, increasing numbers of reformation systems have been proposed for obtaining hydrogen for mobile applications, especially for obtaining hydrogen for the fuel cells of a fuel-cell-operated motor vehicle. This has the advantage that no cumbersome hydrogen storage is required in the vehicle. Instead, the hydrocarbon that is used as the starting material for the reformation reaction, especially methanol, is carried in liquid form, which is very safe. Especially for motor vehicle applications, it is desirable to supply hydrogen from the reformation system in sufficient quantities as soon as possible after the vehicle and the hydrogen producing reformation system are started, so that the fuel cell system can deliver the required driving power for the vehicle correspondingly promptly.
Since most reformation reactions that supply hydrogen proceed at a temperature that is higher than room temperature, as for example, the hydrogen reformation of methanol that is particularly important for use in vehicles, it is already known to heat the reformation reaction chamber at least during cold-start phases. Thus, the system disclosed in U.S. Pat. No. 4,981,676 has a gas burner unit in which a suitable fuel/air mixture supplied to it is burned with an open flame on the outside of a cylindrical exterior wall of the reaction chamber, which is in the form of a cylindrical ring. The reaction chamber is delimited radially inward by the hydrogen separating membrane in the form of a hollow cylinder. As an alternative to burners with an open flame, it is known to heat the reaction chamber by using a catalytic flame-free combustion process or by subjecting it to the flow of a hot gas or liquid stream generated outside the reactor. Another known technique is to provide an external heat source for the reactor, as disclosed in U.S. Pat. No. 5,674,301.
JP 06-345408 A discloses a methanol reformation reactor of the plate stack type in which a hydrogen removal layer is bounded on both sides by a membrane plate, whose opposite side a reformation catalyst layer abuts in each case. The reformation catalyst layers are abutted on their sides that are opposite the hydrogen removal membrane plate by a combustion catalyst layer. In these combustion catalyst layers, catalytic combustion can be performed in order to heat the reactor and especially the reformation catalyst layers that form the reformation reaction chamber.
The present invention poses as the technical problem to be solved the provision of a hydrogen separating membrane, a methanol reformation system equipped therewith, and an operating method therefor with improved cold start behavior, in which the operating temperature is reached as rapidly as possible.
According to the present invention, the hydrogen separating or permeable membrane is provided on at least one side with a catalyst layer for a specific catalytic combustion process. By subjecting the membrane on this side to a flow of reactants of the catalytic combustion process, the catalytic combustion process occurs, thereby heating the hydrogen separating membrane directly. Thus, in a methanol reformation system according to an embodiment of the present invention, the hydrogen separation stage can be brought very rapidly to operating temperature by means of the direct flameless heating of the hydrogen separating membrane.
If the hydrogen separation stage is integrated with the reactor stage, the reaction chamber will then be heated directly. In addition, depending on the requirement, direct heating of the reaction chamber in one of the conventional ways can also be advantageous. While in the conventional procedure, only the reaction chamber is heated directly, a heat loss of the reformation product gas that passes from the reaction chamber into the hydrogen separation stage occurs during cold start phases because the hydrogen separation stage must initially heat the hydrogen separating membrane which is still cold. This heat loss can advantageously be avoided by heating the membrane directly catalytically according to the present invention. As a result, the system as a whole can be brought to operating temperature more rapidly, something which is desirable in mobile applications, such as motor vehicles operated by fuel cells. It is precisely in this application that it may be necessary to activate the hydrogen separation stage initially in a cooled resting state following a shutoff of the system, which can take place according to the present invention very rapidly and with comparatively little expense.
A hydrogen separating membrane according to an embodiment of the present invention is coated with a catalyst layer that catalyzes a catalytic combustion process for hydrogen and/or methanol. This is especially advantageous for reformation systems that generate hydrogen, such as methanol reformation systems. In methanol reformation systems, the hydrogen or methanol is present because of the reforming system and therefore no additional other fuel needs to be provided for the catalytic combustion process for heating the membrane.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.